Solid oxide fuel cell (SOFC) systems are well known. A typical prior art SOFC system uses reformed hydrocarbon fuels and air to generate electricity, syngas, and heat. Syngas is defined as the tail gas from the stack anodes, which is relatively rich in hydrogen (H2) and water.
The only component in air that is useful for the electrochemical reaction within the fuel cell is oxygen, which comprises about 20%. The majority component in air is nitrogen (N2) which comprises about 80%. Nitrogen represents a large volume of inert gas which must be compressed and heated along with the oxygen when air is provided to the reformer for reforming the hydrocarbon fuel and to the SOFC stack for electrochemical reaction with hydrogen.
It is further known to couple an SOFC system to a gas turbine system in a hybrid configuration for generating electricity wherein the effluents from the SOFC are mixed and expanded in a gas turbine to produce additional electricity. Such a hybrid system has the potential to be an extremely high efficiency system.
Prior art SOFC stacks typically are air cooled. Excess air, well beyond the requirement for electrochemical operation, is used to remove heat generated in the stack. The volume of nitrogen present in an air-supplied SOFC system is useful in helping to cool the stack and manage stack temperatures.
Operating a system with high rates of anode tail gas recycled into either the reformer or into the anode inlet allows lower cathode air flow rates and flatter stack temperature profiles, thus improving power density and system efficiency. Such operation also serves to raise the temperature of the stack post-combustor, thus allowing better matching with the inlet temperature requirements of a gas turbine.
Prior art hybrid SOFC/gas turbine systems are not well-suited to some specialized applications. For example, in marine uses such as in manned or unmanned undersea vehicles, aircraft, or in space craft, the presence of NOx in the exhaust is especially undesirable, as may be a gaseous exhaust of any kind in, for example, clandestine undersea operations. Further, for such vehicles, the potential length of submersion or space travel may be governed by the volumes of fuel and oxidant that can be carried.
What is needed in the art is a hybrid SOFC/gas turbine system that produces no nitrogen or oxides of nitrogen in the exhaust.
What is further needed in the art is a hybrid SOFC/gas turbine system that produces a highly concentrated stream of carbon dioxide (CO2).
What is still further needed in the art is a hybrid SOFC/gas turbine system that produces no gaseous exhaust.
What is still further needed in the art is a fuel cell electric generating system that performs for an extended length of operation on a single fueling of oxidant and hydrocarbon fuel.
It is a principal object of the present invention to generate electricity in a fuel cell system without producing nitrogen or oxides of nitrogen in the exhaust.
It is a further object of the invention to provide such a system wherein the exhaust comprises only water and CO2.
It is a still further object of the invention to provide such a system that performs for an extended length of operation on a single fueling of oxidant and hydrocarbon fuel.